1. Field of the Invention
The invention relates to color editing systems and particularly to systems employing reference pictures.
2. Description of the Related Technology
Color prepress systems are used to prepare color pictures for printing. Color prepress systems generally include color processing to facilitate production of printing surfaces which are then used for the reproduction of pictures. Offset lithography, letterpress, and gravure are the three most common color printing processes. In all these processes, images are conventionally formed by photographing or scanning the original through three different color filters, normally red, green and blue (R, G and B). These three color components are combined in various ways to produce printing plates or the equivalent thereof, which in turn control the amount of each colorant deposited at each point of the reproduction. The problem common to these processes is that the exact combination of colorants required for the print to match the original is not related, in any simple way, to measurements that can be made on the original
The preparation of color pictures for printing involves aesthetic or editorial corrections to compensate for imperfect originals or to achieve some special effect, as well as correction to compensate for properties of the particular inks, paper, and press to be used. In conventional electronic systems, whether or not they are computer-based, aesthetic correction and ink correction are combined together in one "color computer". The operator interface to this module is a large number of knobs (or their computer equivalents), the effects of which are learned through long experience. Most systems provide a convenient way to predict the densities of the final color separations that will eventually be used to make the printing plates. Operators learn to associate these densities with particular colors, in many cases assisted by a color book of printed patches corresponding to a large number of combinations of separation densities. Corrections to achieve the desired color at one point generally affect other colors or colors in other areas of the image or picture. To circumvent the interaction, local color correction uses masks to confine the correction to one object or area of the picture. While good results can be achieved by such conventional systems, they require long experience and are generally slow. For further information on color prepress systems, see A Color Prepress System Using Appearance Variables by William F. Schreiber in the Journal of Imaging Technology, 12:200-211, Aug. 4, 1986 incorporated by reference herein.
In view of the disadvantage of conventional prepress systems, U.S. Pat. No. 4,500,919 to William F. Schreiber proposes an improved color processing system. U.S. Pat. No 4,500,919 is expressly incorporated by reference herein. The basic approach taken in U.S. Pat. No. 4,500,919 is complete separation of implementation of aesthetic choices and compensation for ink characteristics. In this mode the operator decides what the output appearance should be. The implementation of these aesthetic choices disclosed may be achieved using a TV display that accurately represents the appearance of the output page and a system permitting interactive adjustment of the output image in terms of its appearance and not of its inks. The compensation for ink and printer characteristics may be implemented in a system that effects precise compensation for ink, paper and press by a transformation from appearance values to colorant values. The transformation may be implemented by a lookup table (LUT) which permits the computation of colorant amounts on the fly during the output process. Entries for the LUT may be computed using data collected from printed color patches. Since ink correction is fully automatic, the operator need have no knowledge of conventional color correction. Consequently, only a short training period is required to get good results. Key features of this improved system include (1) the interactive adjustment of the output image by the operator in terms of its appearance on the TV monitor to achieve aesthetic correction and (2) a conversion of such image to ink in a manner that makes the printed image match the displayed image.
According to an embodiment of above-described system, the cathode ray tube of a display may be accurately calibrated and the viewing conditions carefully set up to control the state of adaptation of the operator s visual system to make sure that the display and viewing conditions are consistent and correlated with the appearance delivered by the final printed result. Establishing and maintaining expensive, precisely calibrated displays and viewing conditions are extremely difficult, and otherwise inconvenient. Low cost and convenience are extremely important in desk-top publishing and similar uses by consumers. It is therefore desirable to provide a color correction system which may effectively achieve good results without accurate calibration of displays such as cathode ray tubes in a conventional TV monitor or computer display it is a further object to avoid a requirement for careful control of viewing conditions.
In order better to describe the invention and its relation to the prior art, the following terms are defined at the outset:
Tristimulus Values--The amounts of three primary colored lights, which, when added, produce a visual or "colorimetric" match with an original color. Such a set of primaries includes the red, green, and blue phosphor colors of a TV tube, in which case the tristimulus values are called R, G, and B.
Appearance Signals--Values produced by any reversible transformation of RGB. Luminance/chrominance (LC1C2) and luminance, hue, and saturation (LHS) are two common sets.
Color--The specification of a colored stimulus requiring at least three component values.
Luminance--That aspect of a colored stimulus relating to its intensity.
Hue--That aspect of a colored stimulus relating to its color name.
Saturation--That aspect of a colored stimulus relating to its purity or absence of contamination with white.
Chrominance--That aspect of a colored stimulus relating to its hue and saturation. The saturation is approximately the ratio of chrominance amplitude to luminance.
Color Space--A three-dimensional space in which each point corresponds to a color, including both luminance and chrominance aspects. RGB forms such a space. LHS forms a set of cylindrical coordinates in color space. The L-axis is the diagonal of RGB space, so that L=0 where R=G=B=0, and L=max where R, G, and B are max The C1C2 plane is perpendicular to the L-axis in LC1C2 space The hue (angle) and chrominance (amplitude) are polar coordinates in the C1-C2 plane.
Lightness--A non-linear transformation of luminance in which equal increments are equally perceptible.
Density--The negative logarithm, to the base ten, of the reflectance or transmittance of a point in an image. In the case of colored inks or dyes, the density is measured through an appropriate color filter. The density is approximately proportional to the quantity of ink laid down. CMYK refer to the densities of cyan, magenta, yellow, and black ink normally used in printing.
Gamut--The range of colors reproducible with a set of inks, lights, or other colorants. The gamut can be conveniently described in terms of a particular region of a color space.
Memory Color--Specific colors which are generally known to viewers and for which most viewers have an absolute memory. The red of a Coca-Cola can or color of a well known sports team's uniforms are examples of memory colors.